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Glossary
.
Accelerator. A device that increases the energy of charged particles
such as electrons and protons.
AGS. The Alternating Gradient Synchrotron, a 33-GeV proton accel-
erator at the Brookhaven National Laboratory.
Annihilatior'. See Antiparticle.
Antimatter. Matter composed of antiparticles, i.e., antiprotons,
antineutrons, antielectrons, instead of, i.e., the ordinary protons,
neutrons, electrons.
Antiparticle. Each particle has a partner, called an antiparticle, which
is identical except that all chargelike properties (electric charge,
strangeness, charm. for example) are opposite to those of the
particle. When a particle and its antiparticle meet, these properties
cancel out in an explosive process called annihilation. The particle
and antiparticle can then disappear and other particles be produced.
Antiprotorl. The antiparticle partner of the proton.
Astrophysics. Physics applied to astronomy and astronomical phe-
nomena such as the evolution of stars and the formation of galaxies.
Asymptotic freedom. The concept that the strong force between
quarks gets weaker as the quarks get close together.
Atom. The smallest unit of a chemical element, approximately
1/100.000,000 centimeter in size, consisting of a nucleus surrounded
by electrons.
Bar~'o''. A type of hadron. The baryon family includes the proton,
213

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214 GLOSSARY
neutron, and those other particles whose eventual decay products
include the proton. Baryons are composed of three-quark combina-
tions.
Beam. A stream of particles produced by an accelerator.
Beauty . See Bottom.
BEPC. A circular electron-positron collider with a total energy up to
6 GeV and high luminosity, under construction near Beijing, China.
Beta decay. The decay of a particle or nucleus by the emission of an
electron or positron through the weak interaction.
Bevatron. A circular accelerator at the Lawrence Berkeley Labora-
tory, Berkeley, California; previously used to accelerate protons up
to 6 GeV and now part of a complex for accelerating nuclei.
BNL. Brookhaven National Laboratory.
Bottom. The distinguishing characteristic of the fifth type of quark,
also called the b quark or beauty quark. Each quark is characterized
by a number of properties including familiar ones like mass and
electric charge, and less familiar ones that were arbitrarily given
names like bottom and charm.
Broken symmetry. The failure of a symmetry principle owing to the
presence of an additional force or phenomenon.
Bubble chamber. A particle detector in which the paths of charged
particles are revealed by a trail of bubbles produced by the particles
as they traverse a superheated liquid. Hydrogen, deuterium, helium,
neon, propane, and Freon liquids have been used for this purpose.
Calorimeter. A particle detector in which the total energy carried by
a particle or group of particles is measured.
Cerenko`, counter. A detector of Cerenkov radiation, which is elec-
tromagnetic radiation emitted by a charged particle when it passes
through matter at a velocity exceeding that of light in that material.
CERN. The European Center for Nuclear Research, located near
Geneva, Switzerland, and supported by most of the nations of
Western Europe.
CESR. The Cornell Electron Storage Ring, an electron-positron
collider with a maximum total energy of 16 GeV located at Cornell
University.
Charm. The distinguishing characteristic of the fourth type of quark,
also called the c quark. Each quark is characterized by a number of
properties, including familiar ones like mass and electric charge, and
less familiar ones that were arbitrarily given names like charm and
bottom.
Charmonium. The family of hadronic particles composed of a charm
quark and an anticharm quark.

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GLOSSAR Y 2 15
Circular accelerator. An accelerator in which the particles move
around a circle many times. being accelerated further in each
revolution around the circle.
Collider. When a high-energy particle collides with a stationary
target' a large portion of the energy resides in the continuing forward
motion. Only a small portion of the energy is available for creating
new particles. In a collider. collisions take place between high-
energy particles that are moving toward each other. In such an
arrangement, most of the energy. is available for creating new
particles.
Colliding-bec~m ac c elevator. See Collider.
Color. A property of quarks and gluons. analogous to electric charge,
which describes how the strong force acts on a quart; or gluon.
Conserl atiO'I lint'. A physical law that states that some quantity or
property cannot be changed in a reaction. The law of conservation of
energy states that the total energy cannot change in a reaction.
Cosmic rays. Energetic particles such as protons that come from
outside the Earth's atmosphere.
Cosmolo
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216 GLOSSARY
.
particles. This force is intermediate in strength between the weak
and strong force. The carrier of the electromagnetic force is the
photon.
Electron. An elementary particle with a unit negative electrical
charge and a mass 1/1840 that of the proton. Electrons surround an
atom's positively charged nucleus and determine the atom's chemi-
cal properties. Electrons are members of the lepton family.
Electron volt. The amount of energy of motion acquired by an
electron accelerated by an electric potential of one volt: MeV,
million electron volts; GeV, billion electron volts; TeV, trillion
electron volts.
Electroweak force or interaction. The force and interaction that
represents the unification of the electromagnetic force and the weak
force.
Elementary particle. A particle (piece of matter) that has no other
kinds of particles inside of it and no subparts that can be identified.
Hence the simplest kind of matter.
Elemerztary-particle physics. The area of basic science whose goal is
to determine and understand the structure and forces of the most
basic constituents of matter and energy.
Fermilab. Fermi National Accelerator Laboratory.
Flavor. A general name for the various kinds of quarks, such as up.
down, and strange. Also sometimes applied to the various kinds of
leptons.
Gamma rays. A term used for the energetic photons that are emitted
in the decay of unstable particles and nuclei.
Gauge theory. A type of general theory of forces, modeled on the
immensely successful modern theory of electromagnetism.
Generation. The classification of the leptons and quarks into families
according to a mass progression. The first generation consists of the
electron and its neutrino and of the up and down quarks. The second
generation consists of the muon and its neutrino and of the charm
and strange quarks. The third generation consists of the tau and its
neutrino and of the bottom and expected top quarks.
GeV. (Giga electron volt) A unit of energy equal to one billion (109)
electron volts.
Gluon. A massless particle that carries the strong force.
Grand unified theory (GUT,. A hoped-for unification of the electro-
weak force with the strong force into a single gauge theory.
Gravitational force or interaction. The weakest of the four basic
forces and the one responsible for the weight of matter and the
motion of the stars and planets.

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GLOSSARY 217
Gra~'iton. A proposed massless particle that is assumed to carry the
gravitational force.
Hadron. A subnuclear. but not elementary' particle composed of
quarks. The hadron family of particles consists of baryons and
mesons. These particles all have the capability of interacting with
each other via the strong force.
HERA. An electron-proton circular collider being constructed at the
DESY laboratory in the Federal Republic of Germany.
Higgs mechanism and particle. A mechanism that may explain the
origin and value of the mass of all or some of the elementary
particles. The mechanism includes a proposed set of particles called
-Higgs particles.
High-energ~ physics. Another name for elementary-particle physics.
This name arises from the high energies required for experiments in
this field.
IHEP. A 76-GeV circular proton accelerator in Serpukhov, USSR.
intermediate vector boson. The general name for the W and Z part-
icles that carry the weak force.
invariance. A property of physical laws and equations such that they
do not change when changes are made in reference or coordinate
systems.
J. A particle made of a c quark (see Charm) and an anti-c-quark. It is
also called the psi particle and is three times as massive as the
proton.
Jet. A narrow stream of hadrons produced in a very-high-energy
collision.
K meson or koon. The next to the lightest meson. It is the lightest
hadron that contains a strange quark.
KKK. A 12-GeV circular proton accelerator at Tsukuba, Japan.
LAMPF. An 800-MeV linear proton accelerator at Los Alamos
National Laboratory, used for nuclear and elementary-particle phys-
iCS.
LEP. A circular electron-positron collider with a maximum design
energy of about 200 GeV being constructed at CERN, Switzerland.
Lepton. A member of the family of weakly interacting particles,
which includes the electron, muon, tau, and their associated neutri-
nos and antiparticles. Leptons are not acted on by the strong force
but are acted on by the electroweak and gravitational forces.
Lifetime. A measure of how long an unstable particle or nucleus
exists on the average before it decays.
Linac. An abbreviation for linear accelerator.
Linear accelerator. In this type of accelerator, particles travel in a

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218 GLOSSARY
straight line and gain energy by passing once through a series of
electric fields.
L``mi'`osit~. A measure of the rate at which particles in a collider
interact. The larger the luminosity the greater the rate of interaction.
Magnet. A device that produces a magnetic field and thus causes
charged particles to move in curved paths. Magnets are essential
elements of all circular accelerators and colliders, as well as of many
particle detectors.
Magnetic mon`'pc~le. A hypothetical particle that would carry a single
north or south magnetic pole. All known particles with magnetic
properties carry both a north and a south magnetic pole.
Mass. The measure of the amount of matter in a particle and an
intrinsic property of the particle.
Meson. Any strongly interacting particle that is not a baryon. Mesons
are composed of quark-antiquark combinations.
Me V. (Mega electron volt) A unit of energy equal to one million
electron volts.
Molec'`le. A type of matter made up of two or more atoms.
Muon. A particle in the lepton family with a mass 207 times that of
the electron and having other properties similar to those of the
electron. Muons may have positive or negative electric charge.
Neutrino. An electrically neutral and massless particle in the lepton
family. The only force experienced by neutrinos is the weak force.
There are at least three distinct types of neutrinos, one associated
with the electron, one with the muon, and one with the taut
Neutron. An uncharged baryon with mass slightly greater than that of
the proton. The neutron is a strongly interacting particle and a
constituent of all atomic nuclei except hydrogen. An isolated neutron
decays through the weak interaction to a proton, electron, and
antineutrino with a lifetime of about 1000 seconds.
Nucleon'. A neutron or a proton.
Nucleus. The central core of an atom, made up of neutrons and
protons held together by the strong force.
Particle. A small piece of matter. An elementary particle is a particle
so small that it cannot be further divided it is a fundamental
constituent of matter.
Particle detector. A device used to detect particles that pass through
it.
PEP. An electron-positron circular collider with a maximum energy
of 36 GeV, at SLAC.
PETRA. An electron-positron circular collider with a maximum en-
ergy of 46 GeV, at DESY, Hamburg, Federal Republic of Germany.

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GLOSSARY 219
Photon. A quantum of electromagnetic energy. A unique massless
particle that carries the electromagnetic force.
Pion. The lightest meson.
Positron. The antiparticle of the electron.
Proton. A baryon with a single positive unit of electric charge and a
mass approximately 1840 times that of the electron. It is the nucleus
of the hydrogen atom and a constituent of all atomic nuclei.
PS. A circular proton accelerator with a maximum energy of 28 GeV
at CERN, Switzerland.
.
Psi. A panicle made of a Cal quark (see Charm) and an anti-c -quark and
three times as heavy as the proton. It is also called the J particle.
Quantum chromodvnamics (QCD). A theory that describes the
strong force among quarks in a manner similar to the description of
the electromagnetic force by quantum electrodynamics.
Quantum electrodynamics (QEDJ. The theory that describes the
electromagnetic interaction in the framework of quantum mechanics.
The particle carrying the electromagnetic force is the photon.
Quantum mechanics. The mathematical framework for describing
the behavior of photons, molecules, atoms, and subatomic particles.
According to quantum mechanics, the forces between these particles
act through the exchange of discrete units or bundles of energy called
quanta.
Quarks. The family of elementary particles that make up the hadrons.
The quarks are acted on by the strong, electroweak, and gravita-
tional forces. Five are known, called up, down, strange, charm, and
bottom. A sixth, called top, is expected to exist.
Relativistic. The term that describes particles moving with velocities
close to the velocity of light.
Scattering. When two particles collide, they are said to scatter off
each other during the collision.
Scintillation counter. A panicle detector in which the passage of a
charged particle produces a flash of light called scintillation light.
That light, when detected, records the time at which the particle
passed through the counter.
SLAC. Stanford Linear Accelerator Center in Stanford, California.
Also refers to the electron linear accelerator there that is being
rebuilt to have a total energy of 50 GeV.
SLC. Stanford Linear Collider, a linear electron-positron collider
with an initial total energy of about 100 GeV being constructed at
SLAC.
SPEAR. A circular electron-positron collider with a total energy of
about 8 GeV at SLAC.

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r
220 GLOSSAR Y
SppS. A circular proton-antiproton collider at CERN that uses the
SPS accelerator there and has a total energy of about 600 GeV.
SPS. A circular proton accelerator with a maximum beam energy of
about 400 GeV at CERN. Switzerland.
SSC. See S``perc o'~d``cting Sniper Collider.
Standard model. A collection of established experimental knowledge
and theories in particle physics that summarizes our present picture
of that field. It includes the three generations of quarks and leptons.
the electroweak theory of the weak and electromagnetic forces. and
the quantum chromodynamic theory of the strong force. It does not
include answers to some basic questions such as how to unify the
electroweak forces with the strong or gravitational forces.
Storc'~,~e ritzy,>. An acceieratorlike machine composed of magnets ar-
ranged in a ring used to store circulating particles or to act as a
collider. Sometimes a synonym for a collider.
Strangeness. The distinguishing characteristic of the third type of
quark' also called the s quark. Each quark is characterized by a
number of properties. including familiar ones like mass and electric
charge and less familiar ones that were arbitrarily given names like
charm and strangeness.
Strange particle. The name given to panicles thought to contain just
one s quark. The remaining quarks in strange particles are either u or
d quarks.
Strong force or interac lion.
.
The short-range force and interaction
between quarks that Is carried by the gluon. The strong force also
dominates the behavior of interacting mesons and baryons and
accounts for the strong binding among nucleons.
Superconducting magnet. See Superconductivity.
Superconducting S`'per Collider (SSC'. A design for a circular pro-
ton-proton collider with a total energy that could be as high as 40
TeV being developed in the United States.
Superconductivity. A property of some metals that when they are
cooled to a temperature close to absolute zero, their electrical
resistance becomes exactly zero. Magnets with superconducting
coils can produce large magnetic fields while keeping size and power
costs small.
Supersymmetrv.
A proposed theory of elementary particles in which
a property of particles called spin is used. In most theories particles
that differ in spin in some ways cannot be related. In this theory such
particles can be related through a new proposed symmetry principle
called supersymmetry.
Symmetry. A general property of many objects and physical systems

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ClOSSARY 221
whereby the object or system appears unchanged when looked at
from different reference frames or coordinate systems. For example,
a tennis ball has spherical symmetry because it always looks the
same to us no matter how we move around it.
Synchrotro'`. A type of circular particle accelerator in which the
frequency of acceleration is synchronized with the particle as it
makes successive orbits.
Synchrotron radiation. intense light or x rays emitted when electrons
move in a circular orbit at relativistic speeds.
Target. The material. often liquid hydrogen. that is struck by the
beam of high-energy particles in some types of elementary-particle
physics experiments.
Tail. An elementary particle in the lepton family with a mass 3500
times that of the electron but with similar properties. There are
positive and negative tau particles.
Tec hail olor. A proposed theory for explaining the masses of particles
that postulates the existence of a new force.
TeV. (Tera electron volt) A unit of energy equal to one thousand
billion ~ 10'') electron volts.
Tevatron. A complex of accelerator facilities and beam lines at
Fermilab. The main facility is a circular proton accelerator with
superconducting magnets (the first large accelerator to use such
magnets) with a maximum energy of 1 TeV. An addition is being
constructed so that this accelerator can be used as an antiprotoo-
proton collider with a total energy of 2 TeV. On completion, this will
be the highest-energy collider in the world.
Top. The distinguishing characteristic of the expected sixth type of
quark, also called the truth quark or t quark. Each quark is
characterized by a number of properties, including familiar ones like
mass and electric charge and less familiar ones that were arbitrarily
given names like bottom and top.
TPC. (Time projection chamber) A particle detector in which the
position of the track of ionized gas left by a charged particle is
detected by the time it takes for the electrons in the gas to move to
the ends of the chamber.
TRISTAN. A circular electron-positron collider with a total energy of
60 to 70 GeV under construction at the KKK laboratory in Japan.
Unified theories. Theories of forces in which the behavior of different
kinds of forces is described by a unified or single set of equations and
has a common origin. For example. the electric and magnetic forces
are unified in the theory of electromagnetism.

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222 GLOSSAR Y
UNK. A complex of high-energy circular proton accelerators and
colliders under construction at Serpukhov, USSR.
Up. The distinguishing property of one of the two lightest quarks
also called the u quark. The up and down quarks form the first quark
generation.
[epsilon. A meson made up of a b quark and an anti-lo-quark. It Is
approximately ten times as massive as the proton.
VEPP4. A circular electron-positron collider with a total energy of
up to 14 GeV at Novosibirsk, USSR.
W. The charged particle that carries the weak force, also called an
~ntermediate-vector boson. Its mass is about 90 times the proton
mass.
Weak force or interaction. The force and interaction that is much
weaker than the strong force, but stronger than gravity. It causes the
decay of many particles and nuclei. It is carried by the W and Z
particles.
X rays. Photons produced when atoms in states of high energy decay
to states of lower energy.
Z. The neutral particle that carries the weak force, also called an
intermediate-vector boson. It is slightly heavier than the W particle,
with a mass about 100 times the proton mass.
.
.